Upset and sobbing at naptime

The last week or so we have been having lots of meltdowns and whacky laughing during our nap time routine!
A couple of times as I have been singing his sleepy song he has had a really sad look on his face like he wants to cry (and is holding it back) and when I put him down he cries like he is scared/upset/hurt himself. I know he is fine though and he does it for around 10 seconds and then goes to sleep. Other times he looks as though he's asleep but about 5 minutes later does the same thing. He still sleeps 2 hours and wakes happy... It doesn't happen at bedtime and is fine through the night though.
He's had a cold over the last week too (he's only ever had 1 up until now and not been sick otherwise) do I blame that? Teething?
I've left him yelling and screaming protesting before but this cry seems so sad :(

Sleep and your preschooler
Even when your child throws off the cot and the all-night feeds, sleep challenges in the preschool years can still keep you wondering. Get tips and advice on how to help your 3 to 4 year old get a great sleep.